Until the discovery of radio galaxies in the early 1950s, radio astronomy was hardly more than an interesting sideline to the primary study of stars and galaxies by optical telescopes. How¬ever, in only a few years rich new vistas of research were opened up. A momentous breakthrough came in about 1954, when British and Australian astronomers managed to construct interferometer telescopes which enabled them to pinpoint a few dozen radio sources with enough precision for optical astronomers to link the radio emitters with visible objects. The then recently-completed 5-m reflector at Palomar turned its penetrating gaze to the strongest radio source, in Cygnus. There Walter Baade and Rudolph Minkowski found a disturbed 16-mag galaxy coincident with strong radio emission. On obtaining a spectrum, their initial suspicions became amply confirmed because the object showed many intense emission lines – a sure sign of unusual activity. Furthermore the lines were displaced to the red end of the spectrum with a redshift of z = 0.057, at the time one of the largest known. It indicates a recession speed of 17 000 km s-1 and a distance of around
170 Mpc

The amazing nature of this discovery lies in the fact that CYGNUS A, as the object is called, is one of the brightest objects in the radio sky, but it is at an immense distance from our Galaxy. The radio luminosity is a fantastic 1038 watts, which is millions of times more energetic than the background of radio emission from our own Galaxy. With the discovery of such titans, extragalactic astronomy was launched on a golden age of expansion.

There are many thousands of radio sources beyond our Galaxy that appear in catalogues drawn up by radio astronomers. Roughly one-third to one-half of these EXTRAGALACTIC RADIO SOURCES coincide in position with galaxies that are visible on deep photographs of the sky. A galaxy that is a strong source of radio emission is known as a RADIO GALAXY ; naturally, with the growth of the subject the variety of galaxies that show radio emission has increased considerably, but we concentrate here on a presentation of the major observed features of radio galaxies.

The radio emission itself has a very characteristic spectrum. For most objects, the FLUX DENSITY at a particular frequency (which is the radio astronomer’s term for the energy received per unit bandwidth) in proportional to the frequency raised to some power, called the spectral index. In mathematical language, this can be expressed as;
S(?) =k ?- ?
where S(?) denotes flux density at frequency ?, K is a constant of proportionality, and ? is the spectral index. The unit of measurement of the flux density is the jansky (Jy), in honour of the pioneering American radio scientist Karl Jansky; it is measured in watt m-2 Hz-1, and 1jy is 10-26 watt m-2Hz-1. Observed flux densities from galaxies in the frequency range 10 MHz to 10 GHz span a range of 10-2 to 105 Jy. Note that in radio astronomy very low energies are in fact being received, despite the great intrinsic luminosity of the radio sources. For example, a flux density of 1 Jy falling on a telescope with a collecting area of 100m2 only gives a signal strength of 10-18 watts with a bandwidth of 1MHz. In are plotted the spectra of a representative sample of radio galaxies, including cases where the spectral index ? is not constant over all the spectrum.

From the radio spectra of galaxies we glean some useful information. Firstly, all the strong radio galaxies have spectra of the type described above, and this indicates that they are probably all energized by the same basic mechanism. Secondly, this power-law spectrum is exactly what we expect when electromagnetic radiation is released by synchrotron emission. In this process the fastest electrons radiate at the highest frequencies, and they are the first to run out of energy (i.e. slow down). This explains why the spectra of some radio galaxies, such as Cygnus A, bend over at high frequencies, where the electrons have become somewhat depleted.

The architecture of radio galaxies is explored with interferometers, because only these can attain the requisite resolving power of a few arc seconds or less. Most radio galaxies are less than one minute of arc in diameter, mainly because they are at a great distance and so subtend a small angular size. The normal methods of displaying data on the structure of a particular radio source are either by means of a contour map, or by processing the data so that it is displayed on a television screen as if it were a photograph. Most strong radio galaxies consist of two large clouds of radio emission symmetrically disposed on either side of the galaxy . Therefore the radio waves are in fact coming from regions of space which are usually well beyond the visible confines of the associated galaxy. The radio clouds may be 10kpc-1Mpc from the galaxy, and may measure 1-50 kpc in diameter. We can see from these values that the radio clouds are often larger than their parent galaxies. Indeed, the very largest radio sources, such as 3C 236 and DA 240, are as big as an entire cluster of galaxies!

At first sight one might think that the predominant double structure of radio galaxies implies that they are surrounded by a ring (or doughnut) of radio emission. That this is not the case is clear from the fact that we never see such doughnuts face on. Therefore we conclude that the radio waves are coming from a pair of cloud-like regions. High-resolution maps show that these clouds often contain compact emission regions, particularly at the periphery of the clouds, and the latter may indicate a reaction between the clouds and invisible matter in extragalactic space. By studying the polarization of the radio emission it is possible to map out the structure of the magnetic fields inside extragalactic radio sources, and this work has tended to show that such structure is rather complex.

The picture of radio clouds emerging from these data is of a vast region of space populated by exceedingly energetic electrons and threaded by a tangled magnetic field. The double structure suggests that these rather exotic materials are somehow cast out of the & optical galaxies, perhaps by immense explosions in the centres. Certainly the activity observed in the nuclei of other galaxy types encourages us to proceed along these lines

When the synchrotron radiation idea is linked to the structural information we can get a notion of the energy requirements of a radio galaxy. This is because the total amount of energy stored up in the magnetic field and the fast electrons is related to the volume of the source and the spectrum of the radiation. We have no way of knowing the detailed distribution of energy in the radio cloud, but the sum total is lowest if we assume (conservatively) that it is equally split between fields and particles. In that case we find stores of energy ranging upwards from 1049 joules to 1052 joules. Recall that the total rest mass energy of the Sun is 2 x 1047 joules, and the total energy that it will release while on the main sequence is roughly 1045joules, and you can appreciate how powerful a radio galaxy is.

Among the radio galaxies deserving a brief mention we may include Cygnus A, Centaurus A, and 30 236. Cygnus A is often taken as the prototype since it is among the most powerful known. Its optical counterpart has a rich spectrum of emission lines, like an over-excited Seyfert galaxy, displaying a redshift of 0.057; this places it at a distance of around 170 Mpc. The two radio clouds are around 50kpc from the optical nucleus, and they both contain a brilliant, condensed, region of radio emission. Radio astronomers in the southern hemisphere have an object of immense angular size, Centaurus A, available for study. It is a puny 4 Mpc from us, but has radio emission spanning 600 kpc of space. Despite this generous expansiveness, it has only one-thousandth the energy of Cygnus A. An interesting feature of Centaurus A is that it contains a further double radio source which has not yet broken free of the galaxy. Finally we briefly mention 3C236 because it appears to be one of the biggest radio galaxies found in the Universe. This object has the usual double structure, centred on a faint galaxy. From the redshift (i.e. distance) of the galaxy and the measured angular size of the radio lobes, astronomers have concluded that 3c 236 stretches for almost 6Mpc! This is nearly ten times the distance from ourselves to the Andromeda nebula.

The principal problems that radio galaxies pose for theorists are: the origin of the immense energy; the translation of this energy into magnetic fields and energetic electrons; and the origin of the double structure. It is widely considered that unknown processes in the central regions of certain elliptical galaxies are at the root of these problems.

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